1. Introduction
This invention relates to electroplating of aluminum and more particularly, to an inexpensive process for the manufacture of aluminum storage discs by an electroplating process and to an improved aluminum storage disc formed by said process.
2. Description of the Prior Art
Electrolytic metal deposition refers to the plating of a metal over a surface by passing a current through an electrolyte where the part to be plated is a cathode and the metal to be plated is typically an anode. Processes and compositions useful therefor are known, are in substantial commercial use and are described in numerous publications. Electroplating formulations and processes are disclosed in Muller et al, Plating on Plastics, Robert Draper Ltd. Teddington, 2nd Ed., 1971, Chapter 17, incorporated herein by reference.
The plating of aluminum for a variety of purposes, including the manufacture of magnetic storage discs, is also well known in the art. An early patent showing a process for the formation of an aluminum storage disc by plating is U.S. Pat. No. 3,466,156, incorporated herein by reference. In this patent, following cleaning of the aluminum disc, the disc is plated by a procedure including the steps of formation of a zinc coating by a zincating process, electroless deposition of a hard, non-metallic nickel layer which serves as a hard base layer and then, electroless deposition of a thin film of cobalt metal as the recording medium. The process is undesirable because it includes multiple steps, uses a zincating solution which is toxic and difficult to dispose of following treatment and provides a recording disc having poor adhesion between the aluminum disc and the subsequently applied metal coating.
More recent processes for plating aluminum include electrical anodizing of the aluminum surface in lieu of zincating. Following anodizing, the anodized coating is electroplated with multiple layers of metals. One patent showing such a process is U.S. Pat. No. 3,886,052, incorporated herein by reference. This patent teaches the preparation of a magnetic recording disc by a process including the steps of electrical anodizing followed by electroplating with copper, polishing the copper plate to provide a smooth surface and then electroplating with a ferromagnetic material, such as a nickel-cobalt alloy.
Another patent showing the plating of aluminum using a step of electrical anodizing of the aluminum is U.S. Pat. No. Re. 28,506 directed to the use of indicia-bearing anodized layers, but not to the formation of magnetic recording discs. In this patent, an aluminum surface is anodized and then plated by an electroless plating process including the steps of contact with a plating catalyst followed by direct electroless metal deposition from any of a variety of electroless metal plating solutions.
A more recent patent showing the plating of aluminum using an anodized coating is U.S. Pat. No. 4,021,592, incorporated herein by reference. This patent is not directed to the formation of magnetic storage discs. In the patent, an aluminum web is anodized and then briefly electroplated with any of a variety of metals to form a discontinuous electroplated surface. Plating is intentionally discontinued before a continuous film of metal is deposited over the anodized layer. The discontinuous film acts as a anchor layer for subsequently applied coatings. Coatings that are suggested for deposition over the anchor layer include photoresists, various plastics, electrolytically plated metals differing from the anchor coat, etc. A process of electroless metal plating is also suggested in this patent. In this process, the aluminum web having the anchor layer is laminated to a plastic substrate with the anchor layer in contact with the plastic. It is intended in the patent that the laminating process embed the anchor layer into the surface of the plastic. In this way, the plastic is secured to the web through the anchor layer. The aluminum layer and the anodized layer are then etched away, leaving the anchor layer embedded in the plastic. The discontinuous isolated deposits of the anchor layer then serve as nucleating sites for electroless metal deposition.
From the above discussion, it can be seen that there are many processes available for the plating of aluminum for a variety of purposes, including the formation of magnetic recording discs. However, all processes involve multiple steps and improved adhesion between an aluminum substrate and a deposited metal is still desired.